Stress from use and/or environmental factors cause cumulative damage to elements of devices. For example, solder joints of an electronic device suffer cumulative damage due to use and environmental conditions. The cumulative damage is often referred to as fatigue. The environmental conditions include many factors such as temperature, humidity, air pressure, etc. For example, temperature changes cause stress to solder due to differences in expansion rates of the solder and the material to which the solder is affixed. The manner in which a device is used significantly affects the rate of damage to the elements. For example, an electronic device that is used constantly will suffer damage at a different rate than one that is used infrequently. During the lifetime of the device, the cumulative effect of the damage can cause a failure of one or more elements, such as a solder joint failing.
Accelerated stress testing is a way to simulate, in a very compressed period, the cumulative effect of the stress that elements of the device would suffer over a longer period of time. The stresses that the elements would experience over its lifetime are accelerated by applying high levels of stimuli over a short time period. High levels of stimuli include temperature cycling, thermal shock, and vibration. For example, the device may be subjected to thousands of stress cycles over a short period of time. As a particular example, a widely used accelerated stress test is the accelerated temperature cycle (ATC) test. One way to perform an ATC is to place a device or printed circuit assembly inside a chamber and cycle the temperature between two extremes. Failure mechanisms such as fatigue, corrosion, and metallic growth caused by migration of ions can be simulated by accelerated stress testing. Accelerated stress testing allows the fatigue suffered by an element of a device over a long period of time to be simulated in a relatively short time.
A drawback with accelerated stress testing is that it is very difficult to know how to construct the accelerated stress test. For example, it is difficult to determine an appropriate number of stress cycles to which a particular device should be subjected. There are generic standards that define how many accelerated test cycles are required to test a very broad class of devices. Typically, the generic test standards err on the side of being too rigorous.
A drawback of generic standards for accelerated tests is that if the standard is too rigorous, the device needs to be over-designed to pass the test. Over-designing may lead to added cost, added complexity, and longer product development times. Another drawback of generic standards is that if the standard is too lenient, the device will not be adequately tested.
Moreover, generic standards do not factor in the particular design of the device. Further, generic standards do not factor in how the device is actually going to be operated in the field. Thus, the generic accelerated stress tests may actually subject the device to far more or far less damage than the particular device would receive over its lifetime in its particular environment and use.
Because of these and potentially other drawbacks, this approach does not provide wholly satisfactory results. Consequently, an improved mechanism for testing the reliability of devices is needed.